A. Field of the Invention
The invention relates to a method for stabilizing semiconductor degas temperature. In particular, the invention relates to a method for stabilizing the wafer temperature inside the degas chamber of semiconductor manufacturing equipment.
B. Description of the Prior Art
Physical vapor deposition(PVD) technology is generally applied to the process of metal sputtering deposition in manufacturing semiconductors. If the wafer temperature is precisely controlled in sputtering equipment, the wafer can be deposited with a film of desired composition and crystalline structure. Referring to FIG. 1, sputtering equipment comprises a set of robots 10 for transferring wafers, one buffer chamber 20, two cassette loading chambers 30, two degas chambers 40 and four sputtering chambers 50. A wafer is transferred through buffer chamber 20 and placed into a degas chamber 40 by using a robot 10. The wafer is oriented and degassed in the degas chamber 40. Then, the wafer is transferred to a sputtering chamber 50 for sputtering by using a robot 10.
After the degassing process of a first wafer of a batch of unprocessed wafers is finished, a second wafer is degassed and the other wafers are also degassed subsequently until the whole batch of wafers are degassed. The initial temperature of a wafer is defined as the wafer temperature at which the pedestal starts to heat the wafer when the wafer is transferred into the degas chamber. In conventional degassing methods, the initial temperature of the first wafer after being transferred into the degas chamber is low since the degas chamber has been idled for a period of time. Since the temperature of the degas chamber 40 is controlled in accordance with an open-loop controlling method, the low initial temperature makes it difficult to raise the temperature of the first wafer after degassing to a setting value. This phenomenon is known as xe2x80x9cfirst wafer effectxe2x80x9d.
Besides, in conventional degassing methods, the temperature of the degas chamber increases after the degassing process of the first wafer. This causes the initial temperature of the second wafer to become much higher than that of the first wafer. Moreover, the temperature after the degassing process of the second wafer is higher than that of the first wafer. Similarly, the initial temperature of the third wafer is higher than that of the second wafer and the temperature after the degassing process of the third wafer is higher than that of the second wafer. After the degassing process of the whole batch of wafers, the temperature of the last wafer will be much higher than that of the first and second wafers. Such a continuous degassing process causes a so-called xe2x80x9ctemperature accumulated effectxe2x80x9d in which the initial temperature, the final temperature and the highest temperature of the wafers increase wafer by wafer.
Thus, the temperature of the whole batch of wafers deviates from the setting value after degassing. This temperature error will affect the grain size, the lattice structure and the roughness of films when the wafers are sputtered in the sputtering chamber and have a certain effect on the wafer structure e.g., the wafer profile which is defined in the precedent process.
FIG. 2 shows the output power curve of an electrical heater according to a conventional degassing method. The X-axis and Y-axis of the diagram represent time and the ratio(%) of the output power to the saturated output power of the electrical heater, respectively. Hereinafter, the ratio is represented as power rate. The method comprises two steps to heat wafers. Before t0, the degas chamber is in an idle status and this status is called an idle baking step. The power rate of the electrical heater in the idle baking step is set as 7%. During time period t0xcx9ct1, a first degassing step is performed. The time period of the first degassing step is set at 30 seconds and the power rate of the electrical heater is set at 30%. During time period t1xcx9ct2, a second degassing step is performed. The time period of the second degassing step is set at 40 seconds and the power rate of the electrical heater is set at 60%. The temperature of the wafer is defined as Tab, where xe2x80x9caxe2x80x9d represents the order of the wafer being transferred into the degas chamber, xe2x80x9cbxe2x80x9d represents the timing of baking, xe2x80x9cb=0xe2x80x9d represents the timing at the end of the idle baking step, and xe2x80x9cbxe2x96xa10xe2x80x9d represents the timing at the end of the first degassing step or the second degassing step. The desired temperature of wafer is set at xe2x80x9cTxe2x80x9d. Since the degas chamber has been idled for a period of time before the first wafer is transferred into the degas chamber, the temperature T11 after the first degassing step is low. Although the power rate of the electrical heater is raised from 30% to 60%, the temperature T12 after degassing still can not reach T. When the second wafer is transferred into the degas chamber 40, the degas chamber door 60 is open. Thus, the degas chamber temperature as well as the initial wafer temperature of the second wafer, is slightly lower than T12 but higher than T10. Therefore, the temperature T22 after degassing is higher than T12 and is close to T. Similarly, T30 is higher than T20, and T32 is higher than T22. Since the temperature is controlled in accordance with an open-loop method, a great temperature difference between the temperature of the last wafer and T12, T22 is caused, and there will be a substantial error between the temperature after degassing of each wafer and the setting value.
An object of the present invention is to provide a method for solving the problems mentioned above. In accordance with the present invention, the method comprises the following steps:
idle baking step: raising the power rate of the electrical heater to 7%xcx9c9%, preferably 9%.
first degassing step: setting the power rate of the electrical heater to 9%xcx9c15%, preferably 9% and prolonging the heating time t0xcx9ct1 maintain the temperature of the wafer after the first wafer at a certain range.
second degassing step (major ramp up step): raising the power rate of the electrical heater to 60%xcx9c80%, preferably 70% and heating the wafer to the setting value T.
third degassing step (stabilizing temperature control step): reducing the power rate of the electrical heater to 5%xcx9c15%, in order to avoid overshoot in temperature control and to stabilize temperature control.
By means of the method of the present invention, the first wafer effect and the temperature accumulated effect of the wafers after the first wafer can be reduced.